The real problem here, is it seems your paying a lot for bandwidth. Bandwidth is cheap and almost non-cost existent, even though your residential ISPs have limits. The funny thing is, the lower end facilities are generally the ones who charge high like this. I think it's time to just switch hosts. Unless, you're paying for a "managed host", which really isn't all what you think.
Your monthly bandwidth is not relative to other businesses your size, it's personalized to the size of your site and the uniqueness. There are a lot of factors, like amount of media, number of pages, number of converting visitors, caching, and crawlers. I would get in touch with a good third party developer, to go through and make sure your browser caching and page size is optimal. If you still want to go the route of trying to save bandwidth.
As I said, the easiest thing would find a host that best fits your needs. I ran a pretty large site at one time, at HostMonster, and when I went to 150GB one month, I didn't pay a dime more. Although, I did have problems with CPU throttling. You might want to check out http://www.blahertech.com/landing.php